Photos in the generic template that aren't 1.91 (width) to 1 (height) aspect ratio will be scaled or cropped. It’s the same ratio across Messenger and Facebook chat box.
Recommended image formats are JPG and PNG.
Recommended resolution: Upload the highest resolution image available that meets ratio requirements.
Recommended aspect ratio is between 9:16 to 16:9, but crops to 1.91:1 with link
Images that consist of more than 20% text may experience reduced delivery.
Recommended Image Dimension: at least 1,200 x 628 pixels
Minimum Image Dimension: at least 254 x 133 pixels